Many houses (and other buildings) have exteriors faced with lap siding which is topped with a trim band. In conventional constructions of this character moisture is apt to penetrate between the trim band and the exterior wall sheathing (or sub siding) and migrate downwardly between the sheathing and the siding. This trapped moisture can lead to rotting of the trim bands and the siding and, perhaps, other building components.
Trim bands as heretofore constructed also tend to be expensive, both in terms of material cost and in the labor required to install them.